THE fundamental idea of the screen-plate was conceived by Ducos du Hauron in 1867. It belongs to the additive processes, and the principle is similar to a particular school of painting, in which small areas of pure pigments are juxtaposed to produce a given color effect, when seen at a certain distance. Thus, if we wash paper with rose Bengal, which is a bright crimson aniline dye, and then apply a wash of malachite green, the result will be a dirty brown; but if instead of superimposing the colors we use them in small areas side by side, the result when examined at a little distance is a yellow. Exactly in the same way blue-violet and green on top of one another would give a dirty olive, but juxtaposed in minute dots we obtain a pure blue; and the violet and red in the dotted form will give a pure crimson. The idea of Ducos du Hauron lay dormant for many years, until in 1892 J. Joly, of Dublin, and J. W. Mc-Donough, of Chicago, conceived methods of putting it into practice, and for this purpose ruled glasses with fine red, green and blue lines, about 200 to 300 to the inch. Such a screen was placed in contact with a panchromatic plate, exposed, and developed. From the line negative thus obtained ordinary silver transparencies were made and bound up in register with similarly ruled screens, and the results were excellent in color. Commercially, the process was a failure, because of the cost of ruling the plates, and the want of a really satisfactory panchromatic plate. In 1007 the Lumiere Co., of Lyons, France, introduced the autochrome plate, in which the small color elements were obtained by sifting potato starch to obtain grains of approximately the same size, staining up three portions with the necessary dyes, then mixing and sifting over tacky glass until a grey surface was obtained, any little interstices between the starch grains being filled up with a black powder. This film was protected with an insulating varnish and then a panchromatic emulsion 'applied. A great many patents have been taken out for the preparation of similar plates and a few have been introduced commercially, but so far as the author is aware the autochrome and the Paget plate are the only two that survive.
Before dealing with the practical working of the process, it may be as well to explain how the colors are formed, and for this purpose a greatly enlarged and purely schematic section of a screen-plate is shown in Fig. 17. On the extreme right are the colors, or the colored lights reflected from the object, which meet first the glass plate G, then pass through the screen elements S, to the emulsion film E.
It will be seen that red light can pass through the red element R only, so that the emulsion is affected by it at this point only and is reduced by the developer, giving the black patch shown. The same argument applies to the green and the blue elements; in each case we obtain the black silver deposit only under the corresponding screen element. In the case of yellow, the light passes through both the red and green elements, therefore, we have these blackened in the negative emulsion. The blue-green and crimson likewise pass light of two colors. White light passes through all three elements, therefore the emulsion is completely and equally reduced under the three. In the case of black, that is, no light, there can be no action, hence there is no silver deposit, and in the case of grey, which is a mixture of black and white, we again have equal action but in a lesser degree. It would be possible to take any number of colors and treat them in the same way, but it will be sufficient to take two other colors, yellowish-orange and brown. In the former case, the red is in full strength mixed with some green, therefore, there is full action under the red and some under the green element; in the case of brown, which is actually nothing more than reddish-orange mixed with black, there is also action under the corresponding elements. Assuming that our diagram thus represents the result of exposure and development, if we should immerse the plate in a fixing bath, it is obvious that we would actually have a negative in the complementary colors of the subject; thus, reading down, the colors shown would be blue-green, crimson, red, green, black, white, grey, bluish-green and a pale greenish-blue. If in any way we should make a positive from this negative and place it in contact with the same screen elements, we should obtain the same colors as those of the object, as shown on the left of Fig. 17. It is unnecessary to follow out the formation of the colors in the positive, as it is obvious from the diagram. This is the whole theory of the screen-plate process.
Everyone knows that it is extremely easy to distinguish small objects as distinct when one is fairly close to them. As one of the very simplest examples, we may take the case of three trees; standing within about twenty feet, we can not only distinguish each branch but also each leaf, and can tell from the shape of the latter whether they (the trees) are elms, oaks or beeches. But at a distance of a mile we no longer see anything but a general green mass, and it would probably be difficult to name the trees. Exactly the same applies to small objects: at the distance of normal vision, about ten inches, one can distinguish quite plainly dots of black and white of a diameter of about 1/50 inch. At a distance of ten feet the dots appear to be merely a grey mass, and as we reduce the size of the dots, or elements, as we have called them, the shorter becomes the distance at which they can be separated, until they become so small that a microscope is required to separate them. In order, therefore, for a patch of color to appear uniform on a screen-plate, the elements have to be very small and in the case of the autochrome they are about 0.015 mm (0.0024 inch), therefore they are not visible at the distance of normal vision.